Gargamelle

Gargamelle was a heavy liquid bubble chamber detector in operation at CERN between 1970 and 1979. It was designed to detect neutrinos and antineutrinos, which were produced with a beam from the Proton Synchrotron (PS) between 1970 and 1976, before the detector was moved to the Super Proton Synchrotron (SPS). In 1979 an irreparable crack was discovered in the bubble chamber, and the detector was decommissioned. It is currently part of the microcosm exhibition at CERN, open to the public. Gargamelle was a heavy liquid bubble chamber detector in operation at CERN between 1970 and 1979. It was designed to detect neutrinos and antineutrinos, which were produced with a beam from the Proton Synchrotron (PS) between 1970 and 1976, before the detector was moved to the Super Proton Synchrotron (SPS). In 1979 an irreparable crack was discovered in the bubble chamber, and the detector was decommissioned. It is currently part of the microcosm exhibition at CERN, open to the public. Gargamelle is famous for being the experiment where neutral currents were discovered. Presented in July 1973 neutral currents were the first experimental indication of the existence of the Z0 boson, and consequently a major step towards the verification of the electroweak theory. Gargamelle can refer to both the bubble chamber detector itself, or the high-energy physics experiment by the same name. The name itself is derived from a 16th-century novel by François Rabelais, The Life of Gargantua and of Pantagruel, in which the giantess Gargamelle is the mother of Gargantua. In a series of separate works in the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with a theory that unified electromagnetic and weak interaction between elementary particles — the electroweak theory — for which they shared the 1979 Nobel Prize in Physics. Their theory predicted the existence of the W± and Z0 bosons as propagators of the weak force. W± bosons have electric charge, either positive (W+) or negative (W−), the Z0, however, has no charge. Exchange of a Z0 boson transfers momentum, spin, and energy but leaves the particle's quantum numbers unaffected — charge, flavor, baryon number, lepton number, etc. Since there is no transfer of electric charge, the exchange of a Z0 is referred to as 'neutral current'. Neutral currents were a prediction of the electroweak theory. In 1960 Melvin Schwartz proposed a method of producing an energetic neutrino beam. This idea was used by Schwartz and others in an experiment in 1962 at Brookhaven, which demonstrated the existence of the muon and electron neutrino. Schwartz shared the 1988 Nobel Prize in Physics for this discovery. Prior to Schwartz' idea weak interactions had been studied only in the decay of elementary particles, especially strange particles. Using these new neutrino beams greatly increased the energy available for the study of the weak interaction. Gargamelle was one of the first experiments that made use of a neutrino beam, produced with a proton beam from the PS. A bubble chamber is simply a container filled with a superheated liquid. A charged particle travelling through the chamber will leave an ionization track, around which the liquid vaporizes, forming microscopic bubbles. The entire chamber is subject to a constant magnetic field, causing the tracks of the charged particles to curve. The radius of curvature is proportional to the momentum of the particle. The tracks are photographed, and by studying the tracks one can learn about the properties of the particles detected. The neutrino beam which travelled through the Gargamelle bubble chamber did not leave any tracks in the detector, since neutrinos have no charge. Interactions with neutrinos were therefore detected, by observing particles produced by the interactions of the neutrinos with the constituents of matter. Neutrinos have extremely small cross sections, a proxy for stating that the probability of interaction is very small. Whereas bubble chambers typically are filled with liquid hydrogen, Gargamelle was filled with a heavy liquid — CBrF3 (Freon) — increasing the probability of seeing neutrino interactions. The domain of neutrino physics was in rapid expansion in the 60's. Neutrino experiments using bubble chambers were already running at the first synchrotron at CERN, the PS, and the question of the next generation of bubble chambers had been on the agenda for some time. André Lagarrigue, an esteemed physicist at the École Polytechnique in Paris, and some of his colleagues, wrote the first published report, dated 10 February 1964, proposing the construction of a heavy liquid chamber to be built under the supervision of CERN. He formed a collaboration consisting of seven laboratories: École Polytechnique Paris, RWTH Aachen, ULB Bruxelles, Istituto di Fisica dell'Università di Milano, LAL Orsay, University College London and CERN. The group met in Milan in 1968 to list the physics priorities for the experiment: today Gargamelle is famous for its discovery of the neutral currents, but while preparing the physics program the topic was not even discussed, and in the final proposal it is ranked as fifth in priority. At the time there was no consensus around the electroweak theory, which might explain the list of priorities. Also, earlier experiments looking for neutral currents in the decay of the neutral kaon into two charged leptons, had measured very small limits of around 10−7. Due to budgetary crisis, the experiment was not approved in 1966, contrary to what was expected. Victor Weisskopf, Director General at CERN, and Bernard Grégory, Scientific Director, decided to commit the money themselves, the latter offering a loan to CERN to cover the instalment due for 1966. The final contract was signed on 2 December 1965, making this the first time in CERN's history that an investment of this kind was not approved by the Council, but by the Director General using his executive authority. The Gargamelle chamber was entirely constructed at Saclay. Though the construction was delayed by about two years, it was finally assembled at CERN in December 1970, and the first important run occurred in March 1971.